1. Field of the Invention
The present invention relates to a voltage-controlled oscillator that oscillates at a frequency according to a control voltage, and relates also to an integrated circuit device provided with such a voltage-controlled oscillator.
2. Description of the Prior Art
Satellite broadcast tuners, cable television tuners, and the like incorporate a PLL (phase-locked loop) synthesizer circuit as a means for controlling the frequency of the local signal, and its oscillation source is typically realized with a voltage-controlled oscillator that oscillates at a frequency according to a control voltage. In a case where the frequency of the local signal needs to be varied over a wide range, the oscillation source of a PLL synthesizer circuit is realized with a voltage-controlled oscillator having a plurality of voltage-controlled oscillation circuits switchably connected in parallel with one another, each voltage-controlled oscillation circuit oscillating in a different frequency range (see Japanese Patent Application Laid-Open No. S58-136142).
It is true that, with the voltage-controlled oscillator disclosed in the patent publication mentioned above, it is possible to vary the frequency of the local signal over a wide range without unnecessarily widening the variable oscillation frequency range of each voltage-controlled oscillation circuit. This helps minimize lowering of the Q value of the resonance circuit that is included in each voltage-controlled oscillation circuit, and thus helps achieve satisfactory phase noise characteristics.
However, the voltage-controlled oscillator configured as described above has the disadvantage that the output levels of the individual voltage-controlled oscillation circuits differ from one another, causing the output level of the local signal to vary according to the frequency. This may pose a problem in a case where the succeeding-stage circuit requires the local signal to be fed thereto at a constant level over the entire frequency range. Even in a voltage-controlled oscillator having only one voltage-controlled oscillation circuit, the output level of the local signal may vary according to the control voltage fed thereto. This may pose a similar problem as described above.
Moreover, in the voltage-controlled oscillator configured as described above, the variable oscillation frequency ranges of adjacent voltage-controlled oscillation circuits are so set as to overlap at their ends to achieve oscillation over a continuous frequency range as a whole. However, no complete study has ever been made of the fact that the variable oscillation frequency ranges of the individual voltage-controlled oscillation circuits vary owing to various factors (such as variation in the supply voltage, variation in the operating temperature, and fabrication variations). As a result, in a case where the voltage-controlled oscillator configured as described above is incorporated in an integrated circuit device, it may be impossible to uniquely decide which voltage-controlled oscillation circuit to select for oscillation at a given frequency.
Now, the problem mentioned above will be explained in more detail with reference to FIG. 7. FIG. 7 is a diagram showing the variable oscillation frequency range of a conventional voltage-controlled oscillator. In the voltage-controlled oscillator shown in this figure, the variable oscillation frequency ranges of its constituent voltage-controlled oscillation circuits VCO1 and VCO2 are, when most deviated on the low side (in a low state), from 90 MHz to 140 MHz and from 130 MHz to 180 MHz, respectively, and, when most deviated on the high side (in a high state), from 110 MHz to 160 MHz and from 150 MHz to 200 MHz, respectively. In this way, in this conventional voltage-controlled oscillator, the variable oscillation frequency ranges of the voltage-controlled oscillation circuits VCO1 and VCO2 are so set as to overlap at their ends in each state to achieve oscillation over a continuous frequency range as a whole.
It is true that, with the voltage-controlled oscillator configured as described above, it is possible to control the oscillation frequency in the range from 110 MHz to 180 MHz in any state. However, when the voltage-controlled oscillation circuits VCO1 and VCO2 are considered individually, their variable oscillation frequency ranges free from the influence of variations are limited to from 110 MHz to 140 MHz and from 150 MHz to 180 MHz, respectively. Thus, for oscillation in the frequency range from 140 MHz to 150 MHz, which to choose between the voltage-controlled oscillation circuits VCO1 and VCO2 cannot be uniquely decided. As a result, the voltage-controlled oscillator configured as described above requires a circuit for checking whether or not each of the voltage-controlled oscillation circuits VCO1 and VCO2 can oscillate at a desired frequency and a circuit for choosing, when one of them is found to be unable to oscillate at that frequency, the other. This increases the circuit scale of and the power consumption by the voltage-controlled oscillator.